In general, an air conditioning unit of a vehicle air conditioning apparatus has a case defining an air passage therein, an evaporator for generating cool air, and a heater core for heating the cool air. The heater core is arranged in the case such that a cool air passage, that is, a bypass passage through which the cool air flows while bypassing the heater core is provided in the case. The air conditioning unit further includes an air mix door disposed upstream of the heater core with respect to an air flow. The air mix door is movable between a maximum heating position in which a heating air passage communicating with the heater core is fully open and the bypass passage is fully closed and a maximum cooling position in which the heating air passage is fully closed and the bypass passage is fully open. That is, the air mix door is configured to control a ratio of the volume of the air passing through the heater core to the volume of the cool air passing through the bypass passage, thereby to control a temperature of air introduced in a passenger compartment of a vehicle.
In such an air conditioning unit, passage resistance of the bypass passage is smaller than that of the heating air passage because the heater core is not provided in the bypass passage. Therefore, when the air mix door is slightly moved from the maximum heating position to a position at which the bypass passage is slightly opened, the cool air tends to largely flow into the bypass passage. As a result, the temperature of air introduced in the passenger compartment is likely to be rapidly reduced.
For example, U.S. Pat. No. 5,878,806 describes an air conditioning apparatus having multiple air mix doors. The air mix doors are disposed in each of a bypass passage and a heating air passage that is in communication with the heater core, respectively. The multiple air mix doors are operated simultaneously in a frame.